The present invention relates to blowing lances, and more particularly to blowing lances used in the refining of a metal by blowing a gas onto the surface of a molten metal bath.
During a refining process, for example during the refining of cast iron or of an iron compound, a refining gas, mostly oxygen, is blown from above onto a molten metal bath.
A blowing lance for blowing from above onto the molten metal bath during a refining process is known. The lance includes a head with nozzles which generate up to 4 or 6 supersonic refining gas jets which impinge on the bath surface at predetermined impact spots. Such a lance is generally characterized by a gas flow rate, which is dependent on the supply pressure of the gas, and by a supersonic gas outflow speed, which is a function of the same supply pressure. In the course of the following description, a lance of this type will be designated by the expression "a conventional lance".
Different techniques have been developed for intensifying the stirring of the metal bath, and bringing continuously new molten metal into contact with an oxidizing gas, avoiding the occurrence of an oversaturation of the oxidizing gas in the bath and for avoiding a local overheating at the impact of the jets.
One such technique is disclosed in Luxembourg Patent No 87 855 (corresponding to U.S. patent application, Ser. No. 803,167, both of which are assigned to Paul Wurth S.A., a Corporation of Luxembourg and which are incorporated herein by reference) which discloses a blowing lance for generating an even number of gas jets where the impingement spots on the surface of the molten metal bath can be rotated in a continuous manner along a circular path during the refining operation. If compared to the above-mentioned conventional lance, this lance distinguished itself by providing a better stirring of the metal bath, by an improved spreading of the oxidizing gas and by a better repartition of the reaction heat in the vicinity of the impact spots of the jets. A head of this lance, according to U.S. patent application Ser. No. 803,167, includes a rotating part or rotor, which is exposed directly to the heat and to the splashes of the bath, but which, presently can not be integrated into the cooling circuit of the lance. As a result, this blowing lance head has a substantially shorter lifetime than the head of a conventional blowing lance, for which the cooling of the static terminal dome section, with fixed tuyeres therein, can easily be achieved.
Another technique, well known in conjunction with the LD-CL Process (CL=Circulating Lance), makes use of an inclined lance body able to circulate around a vertical axis, so as to sweep or scan the surface of the bath with one jet or with a plurality of jets. This LD-CL lance shows advantages which are similar to those mentioned for the lance with rotating jets of U.S. patent application Ser. No. 803,167. The implementation of the circulating lance requires however important mechanical means as well as a complete transformation of the suspension equipment for the lances.
Luxembourg Patent No 87 353 (corresponding to U.S. Pat. No. 4,993,691), both of which are incorporated herein by reference, discloses an adjustable Laval tuyere which allows, generating within a blowing lance, a supersonic gas flow where the speed and the flow rate are adjustable independently of one another. It is therefore possible to obtain with this device jets of varying hardness (or penetration) for different flow rates.
A conventional lance, equipped with such an adjustable Laval tuyere, provides of course the possibility of increasing the flow rate of the oxidizing gas during the refining operation and to thus intensify the stirring of the bath. However, a conventional lance used in combination with a Laval tuyere has several drawbacks, e.g., during use, an overconcentration of the oxidizing gas in the bath may result and/or a local overheating of the bath at the impingement points of the jets on the bath may also result.